Working at the intersection of food and science, we come across a LOT of scientific jargon and terminology, from chemical processes (see: nixtamalization), to physical phenomena (see: non-Newtonian fluids), to species names (see: Aspergillus oryzae). We’ll be sharing many of these fun words with you in this ongoing series, the Cook’s Science Word of the Week. Get ready to impress at your next cocktail party.

When a food changes color in the kitchen, odds are very good it’s becoming brown. There are two main ways that happens: browning caused by cooking, and browning originating with enzymes present in the food itself. Same color, very different phenomena.

If a recipe says to bake until golden brown, we’re dealing with non-enzymatic browning. This is the delicious type of browning. It happens in a few different ways.

The simplest is caramelization. Well, on a chemical level, caramelization is actually quite complicated, but for the cook it’s simple: heat sugar and it turns brown. When a sugar is heated, its molecules break down and react with each other, creating new compounds, some of which are brown and others of which have delightful new toasty, nutty, buttery flavors.

When you caramelize onions, carrots, or the like, it’s the sugar in the vegetable that’s changing, so the sweeter your vegetable is to start with, the sooner and more thoroughly it will caramelize. Raising the pH with a pinch of baking soda will speed up the process. (Be careful: too much baking soda will make carrots mushy by weakening their cell walls.)

If proteins are present, things get a little more elaborate. When you grill a fish, the sugars present in the fish undergo caramelization, but at the same time, other reactions take place: the renowned Maillard reactions. Maillardization is a totally different kind of browning, which requires two partners: sugar and the amino acids that make up protein. The two come together and set off a dizzying set of reactions and interactions, resulting in, again, both brown molecules and many, many different delicious flavor molecules.

The specifics of what happens during Maillardization, and what flavors are produced, depends on numerous factors, including what other molecules are present. One of the reasons cooked beef tastes the way it does is because the iron in its blood leads the Maillard reactions down certain pathways; the Maillard reactions that happen in fish, in pancakes, in roasting coffee beans, are all very different and result in each of those foods’ differently delectable cooked flavors.

Caramelization and Maillardization can—and usually do—both happen together. In a typical cooking scenario, Maillardization starts first, and then caramelization kicks in at a slightly higher temperature, with each set of reactions contributing its own inimitable character to the food.

They can even happen very slowly and at low temperatures, as in black garlic, which is made by cooking regular garlic at around 140 degrees F/60 degrees C for a month or longer, giving it a unique, savory flavor; or in wine as it develops nutty, toasty notes over years aging at cool temperatures.

Despite appearances, the deep brown-to-blackening when food chars isn’t just a further degree of Maillardization or caramelization, it’s a different set of reactions entirely: pyrolysis and carbonization. At this point, sugars and proteins are rapidly broken down into bitter-tasting carbon compounds. A little char can be a nice accent to a food, but no more than a little.

Now, cut an apple or an avocado in half and walk away for a few minutes. When you return, the cut surfaces have developed a brownish tinge. Quite possibly, somebody snuck in and caramelized them while you were gone, but more than likely it was enzymatic browning instead.

Enzymes are proteins with a particular knack for helping other molecules undergo chemical reactions. We are full of enzymes, and so are plants; they’re essential for biological processes, such as photosynthesis or digestion. In the case of our cut apple, polyphenoloxidase enzymes speed up oxidation reactions between oxygen in the air and compounds called polyphenols in the fruit. As a result of these reactions, brown-colored compounds are produced. The same enzymatic processes are responsible for cut potatoes and beets going black; crushed mint and basil leaves slowly developing a swampy taste; bruised bananas browning and softening; and mushrooms discoloring at the slightest touch. The reactions may be an attempt by the plant to protect its wounded tissue, but the biological story is not yet well understood.

Enzymatic browning is an eternal enemy of cooks and food manufacturers, and a number of techniques have been developed to combat it. The tried-and-true method of treating the cut surfaces with lemon juice works because the lemon’s citric and ascorbic acids are both antioxidants: they limit the oxidation of other molecules.

Preventing oxygen from getting to the injured vegetable (as oxygen is needed for the enzymatic reactions to occur) can also work; for instance by wrapping it in plastic or submerging it in a water bath. Destroying (denaturing) the enzyme with high temperatures, exposure to alcohol, or other means will prevent browning as well; that’s why it’s a good idea to blanch cut fruits and vegetables before freezing them. (The freezing itself slows down the reaction but doesn’t prevent it.) And it’s possible to genetically modify crops so they don’t have as much polyphenoloxidase in the first place: That’s the secret to Arctic Apples, which stay white when they’re cut.

Like any enemy, though, enzymatic browning isn’t all bad: It’s also responsible for the browning and accompanying delicious flavor development that takes place after harvest in tea leaves, vanilla beans, raisins, and other tasty brown foods.

“I trust in nature for the stable laws of beauty and utility.”
— Robert Browning